AN INCREASE in Pseudomonas infections in newborn infants was reported in 1955 and it was suggested that this was primarily related to the increased use of high humidity atmospheres, although "an investigation disclosed no contamination of the supply bottles" of the sterile distilled water which was used in the humidifying equipment.1 The methods and the details of the cultures were not reported. In 1958, Mac-Pherson described methods for testing oxygen humidifiers for contamination and found that the extent of contamination was far more extensive than had been anticipated.2 In 1961, an editorial entitled "Water Bugs in the Bassinet" reemphasized the role of contaminated water in nursery infections, and suggested holding an agar plate in front of the jet of a mist generator as a simple method to test for sterility.3 More recent studies of inhalation therapy equipment using particle-sizing air samplers have reported that heated incubators, which